Many liquids are commonly stored in tanks located underground or otherwise disposed so that the actual liquid surface level cannot be observed directly. Furthermore, environmental protection regulations require that tanks which contain certain products such as gasoline and the like be equipped with means for detecting very low leak rates.
While estimates of liquid level in a tank can, in most cases, be made with a calibrated dip stick or the like, this method is neither accurate nor reliable. Furthermore, it provides no means to compensate for volumetric changes resulting from temperature changes.
More sophisticated instruments have been developed which measure the time required for an acoustic signal to travel from a transducer located at a known location in the tank to the surface of the liquid and return to the transducer by reflection from the surface. As with a calibrated dip stick, the gross volume of liquid can be determined from the measured liquid level if the geometry of the tank is known. However, the velocity of an acoustic signal in a liquid varies with temperature of the liquid. Thus, to determine the actual height of the liquid level, the temperature of the liquid must be known. Unfortunately, the temperature of a liquid in a large storage tank is seldom constant throughout the liquid. Accordingly, in using acoustic methods to obtain accurate distance measurements in a liquid, the temperature of the liquid must be measured at various heights from the bottom of the tank and the vertical of the acoustic signal through each horizontal section of liquid compensated for temperature. This, of course, requires a plurality of temperature measuring devices at precisely known locations. Alternatively, a plurality of reflectors may be placed at fixed known locations with respect to the transducer and the time for an acoustic signal return from each such reflector measured. Thus, if the distance between reflectors is known and the temperature of the liquid between two of such reflectors is known, a temperature compensated average velocity can be calculated to determine the height of the liquid surface. However, this approach has inherent difficulties and limitations. At least one temperature measuring device must be submerged in the liquid and the average temperature calculated from various reflection time measurements. Furthermore, it becomes extremely difficult to distinguish between acoustic signals reflected from the liquid surface and acoustic signals reflected from the fixed reflectors, particularly where the reflector is near the liquid surface.
Various attempts have been made to determine net liquid volume in a tank by measuring acoustic signal velocity through the liquid. Typical of such attempts is U.S. Pat. No. 4,805,453 wherein a complicated signal detection scheme is employed in an attempt to distinguish between reflections from the liquid surface and reflections from a fixed reflector. Temperature compensation is attempted in the typical manner of using at least one submerged temperature sensor and extrapolation of temperature data from actual measurement at a fixed location.
It is well known that actual measurement of the temperature of a liquid is difficult and that temperature-sensing devices submerged in a liquid are expensive, subject to failure and frequently unreliable. Furthermore, the prior art has generally failed to recognize that the average velocity of an acoustic signal from a transducer to a reflective surface is not necessarily equal to true velocity at any point between the active surface of the transducer and the reflector because of errors introduced at the surface of the transducer. For example, cavitation may occur at the fluid/transducer interface. Velocity of the acoustic signal through any vapor or foam formed by cavitation would, of course, be different from velocity through the liquid. Any such effects occurring at the face of the transducer thus introduce errors in the measured elapsed time.